An apparatus for a steam engine is known in the art, for example as disclosed in Japanese Patent Publication No. S58-057014. In the steam engine, working fluid is filled in a fluid container, the working fluid is heated and vaporized by a heating device, and the working fluid is cooled down and liquefied by a cooling device, so that fluid pressure in the fluid container is changed. A piston is displaced depending on such fluid pressure change, to obtain mechanical energy.
It is necessary to increase temperature and pressure of steam to be generated in the fluid container by heating the working fluid, in order to increase an engine performance, namely to achieve a high output and a high efficiency in the above steam engine. In the case that heating temperature is simply increased to obtain a higher engine performance, the fluid pressure in the fluid container may be increased above a saturated vapor pressure of the working fluid during its operation. In such a case, the engine performance is adversely decreased.
Namely, when the heating temperature is increased in the above steam engine, a high temperature steam remains in a heating portion even when the piston reaches at its bottom dead center, at which the volume of the fluid container is maximum. Then, the fluid pressure in the fluid container is increased to its maximum value during a movement of the piston, before the piston reaches at its top dead center.
When the fluid pressure is increased as above, the fluid pressure in the fluid container exceeds the saturated vapor pressure of then heating condition (i.e. the heating temperature), and a part of the steam may start liquefaction to thereby decrease the fluid pressure in the fluid container.
When the part of the steam starts the liquefaction, a negative work is generated at the steam engine. As a result, the high output and high efficiency of the steam engine is prevented.